Methods and apparatuses for generating a video binary signal for a video image having a matrix of pixels with each pixel having a grey scale value are well known in the art. See, for example, U.S. Pat. Nos. 4,817,174; 4,791,676; 4,789,933; 4,760,5541; 4,672,463; 4,517,607; 4,630,307; and 4,498,104.
There are two prior art techniques for setting the discrimination threshold, based upon background processing and foreground processing. In background processing, where the pixel of interest is not near an area of the image having great contrast, such as an edge, the setting of the threshold has been based upon the threshold being a fixed value. In foreground processing, where the edge of an image can be clearly found, the threshold is set dependent upon the grey scale value of the pixels surrounding the pixel of interest. This is called a high frequency operator and is typified by U.S. Pat. No. 4,501,016.
In foreground processing, the threshold is dynamically set and is dependent upon the grey scale value of the pixels surrounding the pixel of interest. In the device disclosed in U.S. Pat. No. 4,501,016, a pixel of interest is compared to a threshold value which is defined as the average of highest and the darkest of the pixels in an array of pixels immediately surrounding the pixel of interest. See, for example, column 5, lines 33-36. Necessarily, this means that the threshold is calculated for each pixel of interest. Furthermore, if the calculator is unable to discern the highest and the lowest values, i.e. the pixels are of an imaging area which is essentially constant grey values, the threshold cannot be used because it is too "noisy". Thus, it can be used only in foreground processing applications.
Thus, there is a need in the prior art wherein for background processing, the threshold can be dynamically set yet, yet does not require an extensive amount of computation.